9/11 Blackswan impact on Options Traders

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After the 9/11 terr0rist attacks in 2001, the U.S. stock markets, including the CBOE (Chicago Board Options Exchange), were closed for: 4 full trading days — from Tuesday, September 11 through Friday, September 14, 2001. All major markets in USA resumed normal trading on Monday Sept 17th 2001

What happened to Options expiring on Friday, Sept 14 2001 ?
In 2001 there was only Weekly Expiry for S&P which was Friday of the Week.

Because the exchanges were closed and no opening prices could be determined, the CBOE and OCC (Options Clearing Corporation) moved the expiration forward: SPX options that were originally set to expire on Friday, September 14, 2001, instead expired on Monday, September 17, 2001 — the day markets reopened.

The SPX settlement value (SET) was calculated using Monday's opening prices instead of Friday’s, since that was the next available data point. This adjustment was coordinated by CBOE and OCC to ensure fair and orderly settlement.

Source:ChatGPT

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What Google Gemini had to say about September 14th 2001 Expiry is this,

While specific official announcements from the CBOE (the primary exchange for SPX options) regarding the exact alternative settlement procedures for the September 14, 2001, expiry are difficult to pinpoint directly from readily available summaries, we can infer the likely process based on how such unprecedented situations are usually handled:

Suspension of Normal Settlement: The standard settlement at the market open on September 14th was impossible due to the closure.

Delayed Settlement based on Reopening Price: It is highly probable that the settlement of these options was delayed and based on the opening prices of the S&P 500 index when the markets eventually reopened on Monday, September 17, 2001. This would be the most logical and fair approach to determine the intrinsic value of the options given the extraordinary circumstances.

Adjustments and Communications: Exchanges would have issued official notices and procedures to inform market participants about the revised settlement process. This would involve:
        Confirming the new "settlement date" (likely September 17th).
        Specifying the exact S&P 500 index value to be used for settlement (the opening value on the new settlement day).
        Providing instructions for clearing and settlement firms to handle the cash adjustments for option holders and writers.